A new outbreak of bird flu hit France's foie gras producers just as a ban on exports outside Europe was about to be lifted in time for the crucial holiday period.

The agriculture ministry said the outbreak of the "highly pathogenic" H5N8 strain of the virus was detected Thursday on a duck farm in the southwestern Tarn region, the heart of the lucrative, though controversial, foie gras industry.

Exports outside the European Union had been suspended after an outbreak a year ago, and producers were waiting for the green light -- which had been set for Saturday -- to resume shipments just in time for the Christmas holidays, when the delicacy is especially popular.

Japan, a top export market for foie gras, banned imports from France last December after the H5N1 strain was detected on 69 farms in southwestern France.

As a result of the fresh outbreak, France will be unable to "recover, as anticipated, its status as (a country) free of bird flu" on Saturday, the ministry said in a statement.

Sales within the EU can continue, however, the ministry said.

It said migratory birds were the likely source of the outbreak. 

Some 7,000 ducks were slaughtered while a further 4,500 had died from illness in the region, officials said.

Authorities later announced a number of confirmed or suspected cases of bird flu in the southwestern regions of Gers, Hautes-Pyrenees and Lot-et-Garonne -- where ducks from the supplier of the Tarn farm had been transported -- leading to around 7,000 further birds being culled.

Producers must now wait another three months for the export go-ahead, as long as no further cases are discovered.

A protection zone has been declared within a three-kilometre (two-mile) radius of the Tarn farm, as well as a 10-kilometre surveillance zone.

Foie gras has become a battleground between animals rights campaigners and defenders of France's gourmet traditions.

Force-feeding of geese and ducks to produce foie gras (fatty liver) has been banned in several countries but is legal in France.

The process fattens the birds to around four times their natural body weight.

The H5N8 variant of bird flu, which also hit duck farmers in the Netherlands last month, is highly infectious for poultry but poses little danger to humans.

The H5N1 strain, however, has killed more than 420 people, mainly in southeast Asia, since first appearing in 2003.